Am I overlooking something, or would that mean it’s super
easy for a rational participant to make money?
It’s more like the stock market brokerages. They just take a fee on each trade and don’t need to give you a spread over the stock price
You have to watch out for resolutions are don’t depend on the truth or could be abused.
Examples of markets to avoid are those that a single individual can manipulate. They could take the most profitable side and corrupt the result.